The present invention relates to injection molding machines for making plastic articles and in particular to those using stack molds.
Conventional injection molding machines have devices attached to the stationary platen to hold the machine nozzle against the mold's sprue bush during injection. These holding devices typically take the form of hydraulic cylinders. They have to be strong enough to resist the separating force between the nozzle and the bush generated by the injection pressure of the plastic developed over the orifice size of the nozzle. Typically, this separating force is on the order of 20 tons.
In the case of a single face mold, the reaction of the separating force is transmitted to the stationary platen via the cylinder and from the platen to the mold sprue bush via the stationary half of the mold. Thus, the 20 ton force does not contribute to the force trying to separate the mold during injection. In the case of a stack mold, the force is transmitted to the stationary platen on the core half of one side of the stack mold and not directly to the sprue bush. This component is attached to the sprue bar which is in turn mounted on the center section of the stack mold. Thus the 20 ton force is added to the separating forces affecting the stationary platen half of the stack mold and is subtracted from those forces affecting the moving half of the stack mold.
The problem may best be understood from the following discussion with reference to FIG. 1 of the drawings. FIG. 1 illustrates an injection molding machine 10 having a stack mold 12 mounted between a stationary platen 14 and a moving platen 16. The stack mold 12 has a sprue bar 18 and a sprue bush 20 mounted to the movable center section or platen 22 of the stack mold. A machine nozzle 24 mounted on an injection unit 26 is maintained in contact with the sprue bush 20 during injection by the hydraulic cylinder devices 28 attached between the injection unit 26 and the stationary platen 14. As a result of this construction, the reaction force of the cylinder devices 28 is added to the separating force A and subtracted from the separating force B. For example, if the mold 12 is clamped in a 300 ton machine by a hydraulic cylinder 100 and cylinders 28 develop 20 tons of force then the net clamping force on the stationary half 30 of the stack mold is 300 tons-20 tons which equals 280 tons and the net clamping force on the moving half 32 of the stack mold is 300 tons+20 tons which equals 320 tons.
This unequal clamping of the stack mold halves causes premature flashing of the stationary half of the stack mold in marginal molding situations where all the clamping force available is required.
This problem has been known for years. It has been resolved by typically oversizing the clamping requirements for stack molds or by using different gate sizes in the two halves of the stack mold to compensate for the different clamping forces. Using an oversize clamp is inefficient and modifying gate sizes has to be done empirically and entails customizing mold components. In some marginal cases even this solution does not work as it reduces the processing window available for molding the part.
Accordingly, it is an object of the present invention to provide an injection molding machine wherein unbalanced clamping forces are avoided.
It is a further object of the present invention to provide an injection molding machine as above which avoids the problem of premature flashing.
These and other objects and advantages will become more apparent from the following description and drawings wherein like reference numerals depict like elements.